How do I append an object from a python script in Blender2.8?
had the same problem! Here is the solution that worked for me:

change the last code snippet to following:

#link object to current scene
for obj in data_to.objects:

   if obj is not None:
      bpy.context.collection.objects.link(obj)


you can also add collections and their content from other blendfiles with data_to.collections:

for col in data_to.collections:
    bpy.context.scene.collection.children.link(col)